A young victim of an apparent cluster bomb attack in Misrata, Libya in April last year. Photograph: Odd Andersen/AFP/Getty Images

Four of Britain's biggest banks and insurance companies have blacklisted a dozen companies that manufacture cluster bombs and landmines, including two of the world's largest defence firms.

The Guardian has learned that major firms such as Lloyds Banking Group (through its investment arm Scottish Widows), Aviva, the UK's largest insurer, and the Co-op have imposed a blanket ban on holding shares in companies that make or supply cluster munitions, purging them from nearly all their share portfolios.

Royal Bank of Scotland has banned all new lending to the same companies, and is now reviewing its defence industry shareholdings. Similar action is being taken by all the firms to clear out shares in anti-personnel landmine manufacturers, following intense pressure from human rights campaigners.

The industry is operating two parallel "stop lists", which cover a dozen arms companies involved in making or supplying cluster bombs and anti-personnel landmines, including the US defence companies Lockheed Martin and General Dynamics, and the South Korean industrial conglomerate Doosan.

The unpublicised but co-ordinated move represents a significant strengthening in the ethical investment and lending policies of the institutions involved, which is expected to put the UK's other major banks, Barclays and HSBC, under pressure to follow them.

The clampdown follows criticisms from human rights campaigners, including Amnesty International and the Dutch group IKV Pax Christi, which last year vigorously attacked the industry for failing to take rapid and comprehensive action on their cluster bomb and landmine investments.

It is understood that at least one of the largest firms has found, in a confidential survey, that a significant majority of its customers were unhappy with investing in makers of cluster bombs.

The four firms collectively have investment portfolios worth nearly £600bn and it is estimated they will have to sell hundreds of millions of dollars in shares across all the firms on the stop lists. Aviva, RBS and Scottish Widows have yet to advise their clients of the move, and have yet to complete their purge.

The main exclusions will be portfolios based on the US stock market and privately held share portfolios, which they manage for clients or act as the nominee, where the client controls the investment policy. Those exclusions could affect at least 10% of the total funds run by Aviva and Scottish Widows.

The US investors and clients of some firms involved have angrily resisted attempts to sell off US defence shares, industry sources said, pointing to a growing gulf between UK institutions and the US over ethical and socially responsible investment policies.

The cluster bombs convention, which came into force in 2010, has now been ratified by 70 states and signed by 43, with the notable exception of the US, China and India. The US has been accused of trying to undermine the treaty by forcing through exclusions in other UN treaties. It has instead banned the export of cluster munitions with a failure rate of more than 1% but insists its forces will continue to use them.

The UK government now bans the manufacture, sale and export of the devices, which are still being made or offered for sale by US, Pakistani, Indian, Singaporean, Chinese and Israeli firms. Several companies that sold cluster bombs at the UK's main arms fair last year were expelled.

The Dutch and Swiss governments are now introducing new laws forbidding their financial institutions from investing in cluster munitions firms – a measure the UK government is continuing to resist.

Roos Boer, a policy adviser for IKV Pax Christi, which is based in Utrecht, said she would carefully study the banks' new stances to see if they were exploiting any loopholes. The group remained sceptical about their exclusion from privately owned investment portfolios.

"If these financial institutions are disinvesting from cluster munitions producers that would be a very positive step," she said. "We're generally seeing a positive trend emerging in the UK, but we urge financial institutions to be comprehensive in drawing up their policies.

"Any exception or loophole which allows funding to go to cluster munitions producers runs counter to the goal of a world without cluster munitions, and it runs counter to the demands of the public."

Aviva, which manages global investments worth £400bn, told the Guardian the company had decided in 2008 that the manufacture of cluster munitions and anti-personnel mines undermined fundamental human rights.

After initially banning shareholdings in firms that made cluster munitions or anti-personnel mines from its own shareholder funds, that policy is now being applied across all its portfolios where it controls investment policies. In 2010, it controlled $65m (£41m) worth of bonds in Lockheed Martin and $67m in Textron.

"The Aviva board has now determined that this exclusion should also be applied to Aviva policyholder funds. We are currently working to implement this decision and will provide an update when this is complete," the firm said.

Bailed-out Lloyds bought $62.5m worth of shares in Lockheed Martin in 2009. Scottish Widows Investment Partnership, which has worldwide investments worth £140bn, and is the main investment business for Lloyds, said: "We are now well advanced in a process of identifying and divesting from overseas companies where there is strong evidence of involvement in activities prohibited by the convention.

"This applies to all funds where SWIP [Scottish Widows Investment Partnership] and other Lloyds Banking Group companies control the investment policy of the fund. We have a similar divestment policy in place for companies that are breaching the Ottawa treaty on anti-personnel landmines."

RBS, more than 80% owned by the taxpayer and whose investment portfolios have shrunk to about £30bn after the banking crisis, banned any new lending or financing of companies linked to the cluster bombs industry last year, after being exposed by IKV Pax Christi.

In 2009, it underwrote or bought $200m in shares and bonds in Lockheed Martin and Alliant Techsystems. It is understood to have begun investigating its smaller shareholdings for any shares in the 12 firms on the stop list.

Co-op Asset Management, which has £16bn in investments and uses a different stop list to its competitors, said it would have divested all of its retail investment products and its own investment funds by the end of April.

"All of our active portfolios are no longer invested in such holdings and no further investments in such companies have or will be made through these funds," a spokesman said. "By the end of this month we will also have divested all of our passive, tracker funds, which are non-retail funds owned by the Co-operative's life fund, from these companies."